Reduce the range-reduction table size from 128 entries down to 64 entries, and

reduce the polynomial's degree from 6 down to 4.

Currently we use a degree-6 minimax polynomial on an interval of length 2^-7

around 0 to compute exp2f. Based on the suggestion of @santoshn and the RLIBM

project (https://github.com/rutgers-apl/rlibm-prog/blob/main/libm/float/exp2.c)

it is possible to have a good polynomial of degree-4 on a subinterval of length

2^(-6) to approximate 2^x.

We did try to either reduce the degree of the polynomial down to 3 or increase

the interval size to 2^(-5), but in both cases the number of exceptional values

exploded. So we settle with using a degree-4 polynomial of the interval of

size 2^(-6) around 0.